1. Field of the Invention
This invention relates to a titanium alloy and a method for the production thereof, and more particularly to a titanium alloy with a mirror polished surface and as a raw material far ornaments and a method for the production thereof.
2. Description of the Prior Art
Titanium alloys are metallic materials which possess numerous advantages including small specific gravity, high strength, and excellent corrosionproofness.
When these titanium alloys are adopted for such ordinary mechanical parts as valves, automobile engine parts, and bicycle parts, they are not required to be finished with a mirror surface Comparable with the attractive appearance of ornaments. For such ornaments as watches, however, the titanium alloys are required to be finished with a mirror surface in addition to being vested with the features mentioned above.
Incidentally, the conventional titanium alloys are highly susceptible of oxidation and deficient in thermal conductivity and, therefore, liable to acquire a high temperature while being mirror polished. The polishing, therefore, entails problems such as grinding burn and discoloration of titanium alloys, excessive wear of polishing tools, and clogging of grindstones used for polishing, for example. Since the mirror finish of these titanium alloys is very difficult, it has been necessary to use such measures as stain finish, hairline finish, overcoating as with glass in the place of mirror finish.
In the ordinary .alpha.+.beta. type titanium alloy, since the .alpha. phase and the .beta. phase which are in a complexed state have different hardness and workability and severally have large grain sizes ranging from 30 to 80 .mu.m, the body phase of the alloy is selectively polished. The titanium alloy is at a disadvantage, therefore, in acquiring large irregularities in the polished surface and failing to Obtain a mirror surface.
As a technique aimed at overcoming this disadvantage, JP-A-02-258,960 discloses a method for the heat treatment of a titanium alloy. This method of heat treatment comprises transforming an .alpha.+.beta. type titanium allay or a .beta. type titanium alloy into a .beta. solid solution at a temperature exceeding the .beta. transformation point, quenching the solid solution to normal room temperature, and further subjecting the quenched mass to a treatment of age hardening at a temperature not exceeding the .beta. transformation point thereby effecting precipitation of a fine .alpha. precipitate within a martensite phase and a .beta. phase throughout the entire surface.
Since this method forms the solid solution at a high temperature, it entails the problem that the solid solution acquires strain from the heat treatment and the product resulting from the heat treatment tends to generate torsion and deformation.
Such parts as watches and other similar ornaments which are small and feature attractive appearance must infallibly preclude such deformations as mentioned above. It has been difficult both technically and economically, however, to provide correction of shape for parts which have sustained deformations during the course of the aforementioned solid solution treatment.
Further, since this method performs the treatment for the formation of the .beta. solid solution at a temperature exceeding the .beta. transformation point, the .beta. grains in the residual .beta. phase tend to grow in grain size and, as a result, the individual .beta. grains manifest difference in property for undergoing polishing due to the difference in crystal orientation of the grains. Thus, the ornamental parts provided by the method of heat treatment under consideration equal in quality of mirror finish to the parts made of austenite type stainless steel. By visual observation, the mirror finishes obtained in such ornamental parts are found to fall short of those obtained in the parts of such hard alloys as stellite.